The state of Mississippi’s bridges are in such bad shape that the governor has ordered at least 102 closed this week.

Mississippi Gov. Phil Bryant, a Republican, issued an emergency declaration, authorizing the Mississippi Department of Transportation workers to shut down the bridges, with the help of state troopers, if necessary. The department began notifying counties Thursday to shut down the bridges within 24 hours or the state would step in, according to officials.

The dilapidated bridges “create extreme peril to the safety of persons and property,” the governor said in the order. If the bridges aren’t closed, the Federal Transportation Administration has threatened to withhold funding to the state, according to Melinda McGrath, MDOT’s executive director.

The move follows an April 5 letter to Mr. Bryant from Brandye Hendrickson, acting administrator of the Federal Highway Administration, which listed bridges that inspectors determined to be unsafe. The state must close unsafe bridges immediately or the administration “will be compelled to follow-up with consequential actions,” she wrote. Mississippi was the only state to receive such a letter recently, according to a spokesman for the U.S. Department of Transportation, which oversees the highway administration.

Mississippi’s bridge problems mirror the nation’s. As of the end of 2017, 54,560 out of 615,002 bridges nationwide were determined to be “structurally deficient” by the Federal Highway Administration, meaning the bridge needed significant repair. The American Society of Civil Engineers has estimated it would cost $123 billion to repair all the nation’s bridges. In March 2017, the society issued its “Infrastructure Report Card”—released every four years—and gave U.S. infrastructure an overall grade of “D+”—below standard.

President Donald Trump has said that fixing the nation’s infrastructure is a priority. But his top infrastructure adviser quit last week and his funding plan—involving $200 billion over 10 years—has little chance of getting through the Republican-controlled Congress before the midterm elections.

Mr. Bryant’s proclamation hits 16 of the state’s 82 counties but also said it would include “other parts of the state” if bridges there are found unsafe. As of April 10, about 540 bridges out of 10,783 in Mississippi were closed, according to Mississippi’s Office of State Aid Road Construction.

Many of the bridges are in rural, less populated areas where the tax base cannot easily handle the cost of repairing older, deteriorating spans, say county officials.

Gary Franks, county administrator of Itawamba County, a rural county of about 24,000 people that was named in the order, said funding is always a problem in his county in the northeast part of the state, but the deterioration of older bridges has started to overwhelm local governments in recent years.

“We just can’t generate a lot of income to build those bridges,” said Mr. Franks, who has been county administrator since 1989. “It’s a common problem.”

Leake County, in the middle of the state, has two bridges on the list that will cost at most about $525,000 total to repair, said Joe Andy Helton, a Democratic county supervisor.

But the county doesn’t have the money for the fixes and the closures cause chaos with people having to reroute miles out of their way to travel, he said.

Mr. Helton said he was frustrated by politicians being afraid to raise taxes—even to pay for basic services like roads and bridges.

“There’s only but one way to fix things on the local, state or federal level and that’s taxes,” he said.

Mr. Bryant’s proclamation comes as the state legislature wrestles with possible ways to fund road and bridge repair.

Marty Wiseman, executive director emeritus of Mississippi State University’s John C. Stennis Institute of Government and Community Development, said members of the GOP-dominated legislature have been reluctant to increase state gas taxes, as other southern states like Tennessee have done, to fund road and bridge repairs.

That reluctance is especially pronounced this year—an election year, he said.

Some legislators have suggested using a state lottery to fund roads, but no clear plan has formalized, he said. Many politicians are waiting to see what federal funding might come from Mr. Trump’s plan for a national infrastructure rebuilding program, Mr. Wiseman said. As a result, “the legislature does seem to be stuck in the mud on this issue,” he said.

Write to Cameron McWhirter at cameron.mcwhirter@wsj.com

Corrections & Amplifications 
President Donald Trump’s infrastructure plan involves the government spending $200 billion over 10 years. An earlier version of this article incorrectly stated $200 million over 10 years. (April 12)